Parameterized codebook subsets for use with precoding MIMO transmissions

ABSTRACT

One aspect of the teachings presented herein provides advantages for sending precoder selection feedback from a transceiver to another transceiver, for use by the other transceiver as precoding recommendations. The transceiver generates two types of precoder selection feedback, where one type uses a smaller signaling payload than the other and therefore provides a distinct reduction in the signaling overhead associated with reporting precoding recommendations. The transceiver uses the reduced-overhead type of reporting when reporting at certain times, or on certain channels, or in response to control signaling. In one example, a UE in an LTE network sends precoder information to an eNodeB on the PUCCH by sending smaller-range index values that index only a subset of precoders, but sends full-range index values when reporting on the PUSCH, which index a larger set of precoders.

RELATED APPLICATIONS

This application claims priority from the U.S. provisional patentapplication filed on 7 Apr. 2010 and identified by Application No.61/321,679, and which is explicitly incorporated herein by reference.

FIELD OF THE INVENTION

The teachings herein generally relate to codebooks and precoding, andparticularly relate to the use of parameterized codebook subsets, suchas may be used to restrict codebook selections for differentMultiple-Input-Multiple-Output (MIMO) modes of operation.

BACKGROUND

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance is inparticular improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and relatedtechniques are commonly referred to as MIMO.

The 3GPP LTE standard is currently evolving with enhanced MIMO support.A core component in LTE is the support of MIMO antenna deployments andMIMO related techniques. A current working assumption in LTE-Advanced isthe support of an 8-layer spatial multiplexing mode for 8 transmit (Tx)antennas, with the possibility of channel dependent precoding. Thespatial multiplexing mode provides high data rates under favorablechannel conditions.

With spatial multiplexing, an information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W_(N) _(T) _(×r), which servesto distribute the transmit energy in a subspace of the N_(T)(corresponding to N_(T) antenna ports) dimensional vector space. Theprecoder matrix is typically selected from a codebook of possibleprecoder matrices, and typically indicated by means of a precoder matrixindicator (PMI). The PMI value specifies a unique precoder matrix in thecodebook for a given number of symbol streams.

If the precoder matrix is confined to have orthonormal columns, then thedesign of the codebook of precoder matrices corresponds to aGrassmannian subspace packing problem. In any case, the r symbols in thesymbol vector s each correspond to a layer and r is referred to as thetransmission rank. In this way, spatial multiplexing is achieved becausemultiple symbols can be transmitted simultaneously over the sametime/frequency resource element (TFRE). The number of symbols r istypically adapted to suit the current propagation channel properties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) andhence the received N_(R)×1 vector y_(n) for a certain TFRE on subcarriern (or alternatively data TFRE number n) is thus modeled byy _(n) =H _(n) W _(N) _(T) _(×r) s _(n) +e _(n)  (1)where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder, W_(N) _(T) _(×r), can be a widebandprecoder, which is constant over frequency, or frequency selective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel matrix H, resulting in so-called channeldependent precoding. This is also commonly referred to as closed-loopprecoding and essentially tries to focus the transmit energy into asubspace which is strong in the sense of conveying much of thetransmitted energy to the targeted receiver, e.g., a user equipment(UE). In addition, the precoder matrix also may be selected with thegoal of orthogonalizing the channel, meaning that after proper linearequalization at the UE or other targeted receiver, the inter-layerinterference is reduced.

In closed-loop precoding for the LTE downlink in particular, the UEtransmits, based on channel measurements in the forward link (downlink),recommendations to the eNodeB of a suitable precoder to use. A singleprecoder that is supposed to cover a large bandwidth (widebandprecoding) may be fed back. It also may be beneficial to match thefrequency variations of the channel and instead feedback afrequency-selective precoding report, e.g. several precoders, one perfrequency subband. This approach is an example of the more general caseof channel state information (CSI) feedback, which also encompassesfeeding back entities other than precoders, to assist the eNodeB inadapting subsequent transmissions to the UE. Such other information mayinclude channel quality indicators (CQIs) as well as a transmission rankindicator (RI).

For the LTE uplink, the use of closed-loop precoding means that theeNodeB selects precoder(s) and the transmission rank. The eNodeBthereafter signals the selected precoder that the UE is supposed to use.The eNodeB also may use certain bitmap-based signaling to indicate theparticular precoders within a codebook that the UE is restricted to use.See, e.g., Section 7.2 of the 3GPP Technical Specification, TS 36.213.One disadvantage of such signaling is the use of bitmaps to indicateallowed or disallowed precoders. Codebooks with large numbers ofprecoders require long bitmaps, and the signaling overhead associatedwith transmitting long bitmaps becomes prohibitive.

In any case, the transmission rank, and thus the number of spatiallymultiplexed layers, is reflected in the number of columns of theprecoder. Efficiency and transmission performance are improved byselecting a transmission rank that matches the current channelproperties. Often, the device selecting precoders is also responsiblefor selecting the transmission rank. One approach to transmission rankselection involves evaluating a performance metric for each possiblerank and picking the rank that optimizes the performance metric. Thesekinds of calculations are often computationally burdensome and it istherefore an advantage if calculations can be re-used across differenttransmission ranks. Re-use of calculations is facilitated by designingthe precoder codebook to fulfill the so-called rank nested property.This means that the codebook is such that there always exists a columnsubset of a higher rank precoder that is also a valid lower rankprecoder.

The 4-Tx House Holder codebook for the LTE downlink is an example of acodebook that fulfills the rank nested property. The property is notonly useful for reducing computational complexity, but is also importantin simplifying overriding a rank selection at a device other than theone that has chosen the transmission rank. Consider for example the LTEdownlink where the UE selects the precoder and rank, and conditioned onthose choices, computes a CQI representing the quality of the effectivechannel formed by the selected precoder and the channel. Since the CQIthus reported by the UE is conditioned on a certain transmission rank,performing rank override at the eNodeB side makes it difficult to knowhow to adjust the reported CQI to take the new rank into account.

However, if the precoder codebook fulfills the rank nested property,overriding the rank to a lower rank precoder is possible by selecting acolumn subset of the original precoder. Since the new precoder is acolumn subset of the original precoder, the CQI tied to the originalprecoder gives a lower bound on the CQI if the new reduced rank precoderis used. Such bounds can be exploited for reducing the CQI errorsassociated with rank override, thereby improving the performance of thelink adaptation.

Another issue to take into account when designing precoders is to ensurean efficient use of the transmitter's power amplifiers (PAs). Usually,power cannot be borrowed across antennas because, in general, there is aseparate PA for each antenna. Hence, for maximum use of the PAresources, it is important that the same amount of power is transmittedfrom each antenna, i.e., a precoder matrix W should fulfill[WW*] _(mm) =κ,∀m  (2)Thus, it is beneficial from a PA utilization point of view to enforcethis constraint when designing precoder codebooks.

Full power utilization is also ensured by the so-called constant modulusproperty, which means that all scalar elements in a precoder have thesame norm (modulus). It is easily verified that a constant modulusprecoder also fulfills the full PA utilization constraint in (2) andhence the constant modulus property constitutes a sufficient but notnecessary condition for full PA utilization.

As a further aspect of the LTE downlink and associated transmitteradaptation, the UE reports CQI and precoders to the eNodeB via afeedback channel. The feedback channel is either on the Physical UplinkControl Channel (PUCCH) or on the Physical Uplink Shared Channel(PUSCH). The former is a rather narrow bit pipe where CSI feedback isreported in a semi-statically configured and periodic fashion. On theother hand, reporting on PUSCH is dynamically triggered as part of theuplink grant. Thus, the eNodeB can schedule CSI transmissions in adynamic fashion. Further, in contrast to CSI reporting on PUCCH, wherethe number of physical bits is currently limited to 20, CSI reports onPUSCH can be considerably larger. Such a division of resources makessense from the perspective that semi-statically configured resourcessuch as PUCCH cannot adapt to quickly changing traffic conditions, thusmaking it important to limit their overall resource consumption.

More generally, maintaining low signaling overhead remains an importantdesign target in wireless systems. In this regard, precoder signalingcan easily consume a large portion of the available resources unless thesignaling protocol is carefully designed. The structure of possibleprecoders and the overall design of the precoder codebook plays animportant role in keeping the signaling overhead low. A particularlypromising precoder structure involves decomposing the precoder into twomatrices, a so-called factorized precoder. The precoder can then bewritten as a product of two factorsW _(N) _(T) _(×r) =W _(N) _(T) _(×k) ^((c)) W _(k×r) ^((t)),  (3)where an N_(T)×k conversion precoder W_(N) _(T) _(×k) strives forcapturing wideband/long-term properties of the channel such ascorrelation, while a k×r tuning precoder W_(k×r) ^((t)) targetsfrequency-selective/short-term properties of the channel.

Together, the factorized conversion and tuning precoders represent theoverall precoder W_(N) _(T) _(×r), which is induced by the signaledentities. The conversion precoder is typically, but not necessarily,reported with a coarser granularity in time and/or frequency than thetuning precoder to save overhead and/or complexity. The conversionprecoder serves to exploit the correlation properties for focusing thetuning precoder in “directions” where the propagation channel on averageis “strong.” Typically, this is accomplished by reducing the number ofdimensions k covered by the tuning precoder. In other words, theconversion precoder W_(N) _(T) _(×k) ^((c)) becomes a tall matrix with areduced number of columns. Consequently, the number of rows k of thetuning precoder W_(k×r) ^((t)) is reduced as well. With such a reducednumber of dimensions, the codebook for the tuning precoder, which easilyconsumes most of the signaling resources since it needs to be updatedwith fine granularity, can be made smaller while still maintaining goodperformance.

The conversion and the tuning precoders may each have a codebook oftheir own. The conversion precoder targets having high spatialresolution and thus a codebook with many elements, while the codebookthe tuning precoder is selected from needs to be rather small in orderto keep the signaling overhead at a reasonable level.

To see how correlation properties are exploited and dimension reductionachieved, consider the common case of an array with a total of N_(T)elements arranged into N_(T)/2 closely spaced cross-poles. Based on thepolarization direction of the antennas, the antennas in the closelyspaced cross-pole setup can be divided into two groups, where each groupis a closely spaced co-polarized Uniform Linear Array (ULA) with N_(T)/2antennas. Closely spaced antennas often lead to high channel correlationand the correlation can in turn be exploited to maintain low signallingoverhead. The channels corresponding to each such antenna group ULA aredenoted H_(/) and H_(\), respectively. For convenience in notation, thefollowing equations drop the subscripts indicating the dimensions of thematrices as well as the subscript n. Assuming now that the conversionprecoder W^((c)) has a block diagonal structure,

$\begin{matrix}{W^{(c)} = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}.}} & (4)\end{matrix}$The product of the MIMO channel and the overall precoder can then bewritten as

$\begin{matrix}\begin{matrix}{{HW} = {\begin{bmatrix}H_{/} & H_{\backslash}\end{bmatrix}W^{(c)}W^{(t)}}} \\{= {{\begin{bmatrix}H_{/} & H_{\backslash}\end{bmatrix}\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}}W^{(t)}}} \\{= {\begin{bmatrix}{H_{/}{\overset{\sim}{W}}^{(c)}} & {H_{\backslash}{\overset{\sim}{W}}^{(c)}}\end{bmatrix}W^{(t)}}} \\{= {H_{eff}{W^{(t)}.}}}\end{matrix} & (5)\end{matrix}$As seen, the matrix {tilde over (W)}^((c)) separately precodes eachantenna group ULA, thereby forming a smaller and improved effectivechannel H_(eff). As such, W^((c)) is sometimes referred to as an“antenna subgroup” precoder. If {tilde over (W)}^((c)) corresponds to abeamforming vector, the effective channel would reduce to having onlytwo virtual antennas, which reduces the needed size of the codebook usedfor the second tuning precoder matrix W^((t)) when tracking theinstantaneous channel properties. In this case, instantaneous channelproperties are to a large extent dependent upon the relative phaserelation between the two orthogonal polarizations.

It is also helpful for a fuller understanding of this disclosure toconsider the theory regarding a “grid of beams,” along with DiscreteFourier Transform (DFT) based precoding. DFT based precoder vectors forN_(T) transmit antennas can be written in the form

$\begin{matrix}{{w_{n}^{({N_{T},Q})} = \begin{bmatrix}w_{1,n}^{({N_{T},Q})} & w_{2,n}^{({N_{T},Q})} & \ldots & w_{N_{T},n}^{({N_{T},Q})}\end{bmatrix}^{T}}{{w_{m,n}^{({N_{T},Q})} = {\exp( {j\frac{2\;\pi}{N_{T}Q}{mn}} )}},{m = 0},\ldots\mspace{11mu},{N_{T} - 1},{n = 0},\ldots\mspace{11mu},{{QN}_{T} - 1},}} & (6)\end{matrix}$where w_(m,n) ^((N) ^(T) ^(,Q)) is the phase of the m:th antenna, n isthe precoder vector index (i.e., which beam out of the QN_(T) beams) andQ is the oversampling factor.

For good performance, it is important that the array gain function oftwo consecutive beams overlaps in the angular domain, so that the gaindoes not drop too much when going from one beam to another. Usually,this requires an oversampling factor of at least Q=2. Thus for N_(T)antennas, at least 2N_(T) beams are needed.

An alternative parameterization of the above DFT based precoder vectorsis

$\begin{matrix}{{w_{l,q}^{({N_{T},Q})} = \begin{bmatrix}w_{1,{{Ql} + q}}^{({N_{T},Q})} & w_{2,{{Ql} + q}}^{({N_{T},Q})} & \ldots & w_{N_{T},{{Ql} + q}}^{({N_{T},Q})}\end{bmatrix}^{T}}{{w_{m,{{Ql} + q}}^{({N_{T},Q})} = {\exp( {j\frac{2\;\pi}{N_{T}}{m( {l + \frac{q}{Q}} )}} )}},}} & (7)\end{matrix}$for m=0, . . . , N_(T)−1, l=0, . . . , N_(T)−1, q=0, 1, . . . , Q−1, andwhere l and q together determine the precoder vector index via therelation n=Ql+q. This parameterization also highlights that there are Qgroups of beams, where the beams within each group are orthogonal toeach other. The q:th group can be represented by the generator matrix

$\begin{matrix}{G_{q}^{(N_{T})} = {\begin{bmatrix}w_{0,q}^{({N_{T},Q})} & w_{1,q}^{({N_{T},Q})} & \ldots & w_{{N_{T} - 1},q}^{({N_{T},Q})}\end{bmatrix}.}} & (8)\end{matrix}$By insuring that only precoder vectors from the same generator matrixare being used together as columns in the same precoder, it isstraightforward to form sets of precoder vectors for use in so-calledunitary precoding where the columns within a precoder matrix should forman orthonormal set.

Further, to maximize the performance of DFT based precoding, it isuseful to center the grid of beams symmetrically around the broad sizeof the array. Such rotation of the beams can be done by multiplying fromthe left the above DFT vectors w_(n) ^((N) ^(T) ^(,Q)) with a diagonalmatrix W_(rot) having elements

$\begin{matrix}{\lbrack W_{rot} \rbrack_{mm} = {{\exp( {j\frac{\pi}{{QN}_{T}}m} )}.}} & (9)\end{matrix}$The rotation can either be included in the precoder codebook oralternatively be carried out as a separate step where all signals arerotated in the same manner and the rotation can thus be absorbed intothe channel from the perspective of the receiver (transparent to thereceiver). Henceforth, in discussing DFT precoding herein, it is tacitlyassumed that rotation may or may not have been carried out. That is,both alternatives are possible without explicitly having to mention it.

One aspect of the above-described factorized precoder structure relatesto lowering the overhead associated with signaling the precoders, basedon signaling the conversion and the tuning precoders W^((c)) and W^((t))with different frequency and/or time granularity. The use of a blockdiagonal conversion precoder is specifically optimized for the case of atransmit antenna array consisting of closely spaced cross-poles, butother antenna arrangements exist as well. In particular, efficientperformance with a ULA of closely spaced co-poles should also beachieved. However, the method for achieving efficient performance inthis regard is not obvious, with respect to a block diagonal conversionprecoder structure.

Another aspect to consider is that, in a general sense, theabove-described factorized precoder feedback may prevent full PAutilization, and may violate the aforementioned rank nested property.These issues arise from the fact that the two factorized precoders—i.e.,the conversion precoder and the tuning precoder—are multiplied togetherto form the overall precoder and thus the normal rules for ensuring fullPA utilization and rank nested property by means of constant modulus andcolumn subset precoders, respectively, do not apply.

Further precoding considerations, particularly in the context of the LTEdownlink, include the fact that the PUCCH cannot bear as large a payloadsize as the PUSCH, for the previously described reasons. Thus, there isa risk of “coverage” problems when a UE reports CSI on the PUCCH. Inthis regard, it is useful to understand that current precoder designscommonly are optimized for transmissions to/from a single UE. In theMIMO context, this single-user context is referred to as a Single UserMIMO or SU-MIMO. Conversely, co-scheduling multiple UEs on the sametime/frequency resources is called Multi User MIMO or MU-MIMO. MU-MIMOis gaining increasing interest, but it imposes different requirements onprecoder reporting and the underlying precoder structures.

SUMMARY

One aspect of the teachings presented herein relates to sending precoderselection feedback from a second transceiver to a first transceiver, foruse by the first transceiver as precoding recommendations. Inparticular, the second transceiver generates two types of precoderselection feedback, where one type uses a smaller signaling payload thanthe other type and therefore provides a distinct reduction in thesignaling overhead associated with reporting precoding recommendationsto the first transceiver. As an example, the second transceiver sendsprecoding recommendations using the reduced-overhead type of reportingwhen reporting at certain times, or when reporting on certain channels,or in response to received control signaling. The smaller-payloadsignaling comprises, for example, restricted-smaller-range index valuesthat index a subset of precoders within a larger set, while thelarger-payload signaling comprises full-range index values that indexthe larger set. The restricted-range index values are also referred toas “smaller-range” index values, to emphasize that they can only index asmaller range of precoders within the larger set.

As a particularly advantageous but still non-limiting example the firstand second transceivers are configured for operation in a wirelesscommunication network operating in accordance with the 3GPP LTEstandards—e.g., the first transceiver is a eNB in the network and thesecond transceiver is a mobile terminal or other item of user equipment(UE). Here, the second transceiver indicates its precodingrecommendations by sending index values as the precoder selectionfeedback, where each index value “points” to one or more precoders in apredefined codebook. When reporting precoder selection feedback on thePhysical Uplink Shared Channel (PUSCH), the second transceiver sendsfull-range index values that span a predetermined set of precoders,e.g., an entire codebook. However, when reporting precoder selectionfeedback on the Physical Uplink Control Channel (PUCCH), the secondtransceiver sends smaller-range index values. While the smaller-rangeindex values cover only a subset of the precoders in the predeterminedset, they are advantageously reported using a smaller payload than usedfor reporting the full-range index values. That is, the smaller-rangeindex values are capable of indexing only a portion of the full set ofprecoders, while the full-range index values are capable of indexingacross the full set of precoders.

With that example in mind, the teachings herein broadly provide a methodin a second wireless communication transceiver of providing precoderselection feedback to a first wireless communication transceiver, asprecoding recommendations for the first transceiver. Here, the terms“first transceiver” and “second transceiver” denote, as a non-limitingexample, a wireless network base station operating as the firsttransceiver and an item of user equipment (UE) operating as the secondtransceiver, wherein the base station precodes certain transmissions tothe UE, based at least in part on receiving precoder selection feedbackfrom the UE, indicating the UE's precoder recommendations.

In any case, the method includes determining channel conditions at thesecond transceiver, and when operating in a first feedback mode,selecting a precoder from a predetermined set of precoders based on saidchannel conditions, and sending a full-range index value for theselected precoder to the first transceiver as the precoder selectionfeedback. However, when operating in a second feedback mode, the methodincludes the second transceiver selecting a precoder from a smaller,predetermined subset of precoders contained within the predetermined setof precoders based on the channel conditions, and sending asmaller-range index value for the selected precoder to said firsttransceiver as the precoder selection feedback. Here, the secondtransceiver uses a smaller signaling payload for sending smaller-rangeindex values, as compared to the signaling payload used to sendfull-range index values.

In at least one embodiment, the second transceiver sends the precoderselection feedback at certain first times on a control channel and atcertain second times on a data channel. The method further includes thesecond transceiver selecting the first feedback mode when sending theprecoder selection feedback multiplexed with data on the same physicalchannel. Conversely, the second transceiver selects the second feedbackmode when sending the precoder selection feedback on the controlchannel. Correspondingly, the first transceiver is advantageouslyconfigured to receive and use (e.g., recognize and respond to) bothtypes of feedback.

In a related embodiment, the teachings herein provide example detailsfor a wireless communication transceiver that is configured to provideprecoder selection feedback to another wireless communicationtransceiver, as precoding recommendations for that other transceiver.The transceiver includes a receiver configured to receive signals fromsaid other transceiver and a channel estimator configured to estimatechannel conditions at the transceiver with respect to signals receivedfrom the other transceiver. The transceiver further includes atransmitter configured to transmit signals to the other transceiver,including signals conveying the precoder selection feedback.Additionally, the transceiver includes a precoding feedback generatorthat is configured to determine whether to operate in a first feedbackmode or a second feedback mode.

When operating in the first feedback mode, the precoding feedbackgenerator is configured to select a precoder from a predetermined set ofprecoders based on the channel conditions and send a full-range indexvalue for the selected precoder to the other transceiver, as theprecoder selection feedback. When operating in the second feedback mode,the precoding feedback generator is configured to select a precoder froma predetermined subset of precoders contained in the predetermined setof precoders. That selection is also based on the channel conditions,but here the second transceiver sends a smaller-range index value forthe selected precoder to the other transceiver. In particular, theprecoding feedback generator is configured to use a smaller signalingpayload for sending smaller-range index values as compared to thesignaling payload used to send full-range index values. As a workingexample, the smaller-range index values can only point precoders in aparticular subset or subsets of precoders within one or more codebooks,while the full-range index values can point to any of the precoderswithin the codebook(s).

As regards the first transceiver, which may be, for example, a networkbase station that implements transmit precoding, the teachings hereindisclose a method wherein the first transceiver receives precodingrecommendations from the second transceiver, in the form of precoderselection feedback. The method includes receiving precoder selectionfeedback from the second transceiver and determining whether theprecoder selection feedback comprises a first type of precoder selectionfeedback including a full-range index value, or a second type ofprecoder selection feedback including a smaller-range index value thatis signaled by the second transceiver using a lower signaling overheadas compared to that used for signaling full-range index values.

According to this method, if the precoder selection feedback is thefirst type of precoder selection feedback, the first transceiveridentifies the precoder recommendation by identifying a precoder from apredetermined set of precoders that is indexed by the full-range indexvalue included in the precoder selection feedback. On the other hand, ifthe precoder selection feedback is of the second type, the firsttransceiver identifies the precoder recommendation by identifying aprecoder from a predetermined subset of precoders that is indexed by thesmaller-range index value included in the precoder selection feedback.Here, the predetermined subset is contained within the predeterminedsubset and comprises, by way of example, every K:th entry in thepredetermined set, where K is some integer value. In any case, themethod continues with the first transceiver determining a precodingoperation for precoding a transmission to the second transceiver basedat least in part on the precoder recommendation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of example embodiments of a first transceiverthat is configured to transmit precoded transmissions to a secondtransceiver.

FIG. 2 is a diagram of one embodiment of a predetermined set ofprecoders having a logical subset defined within it.

FIGS. 3 and 4 are diagrams of one embodiment of full-range andsmaller-range index values, for use in indexing all or a subset of thepredetermined set of precoders shown in FIG. 3.

FIG. 5 is a logic flow diagram of one embodiment of a method for sendingprecoder selection feedback from one transceiver to another transceiver,where the precoder selection feedback transmitted uses a varying payloadsize to signal the recommendations.

FIG. 6 is a logic flow diagram of one embodiment of a method forreceiving and processing precoder selection feedback that includes avariable payload size for signaled precoder information.

FIG. 7 is a block diagram of one embodiment of a precoding circuit, suchas may be implemented in the first transceiver of FIG. 1.

FIG. 8 is a block diagram of one embodiment of a wireless communicationnetwork, wherein precoding restriction signaling and processing astaught herein are used between a base station and an item of userequipment (a “UE”).

DETAILED DESCRIPTION

FIG. 1 depicts a first wireless communication transceiver 10 and asecond wireless communication transceiver 12, referred to forconvenience as transceivers 10 and 12. The transceiver 10 includes anumber of antennas 14 and associated transceiver circuits 16 (includingone or more radiofrequency receivers and transmitters), along withcontrol and processing circuits 18. At least functionally, the controland processing circuits 18 include a precoding controller 20, a feedbackprocessor 22, and one or more memory circuits 24 that store a codebook26 of precoders 28. While the number “28” is used generally as areference in both the singular and plural senses, for referring to oneor multiple precoders 28, suffix designations are used, where helpfulfor clarity, e.g., precoder 28-1, precoder 28-2, and so on.

The second transceiver 12 includes a number of antennas 30 andassociated transceiver circuits 32 (including one or more radiofrequency receivers and transmitters), along with control and processingcircuits 34. At least functionally, the control and processing circuits34 include received signal processing circuitry 36, e.g.,demodulation/decoding circuits, and further include one or moreestimation circuits 38, for estimating channel conditions and/or signalquality.

Further, the control and processing circuits 34 include one or morememory circuits 40, and a precoding feedback generator 42. The memorycircuit(s) 40 store, for example, the same codebook 26 of precoders 28as stored at the transceiver 10. In this manner, the transceiver 12 cansend precoder selection feedback 44 to the transceiver 10 by sending(Precoder Matrix Index) PMI values. The PMI values indicate the codebookindex value of the precoder(s) 28 selected by the transceiver 12, asrecommended for use by the transceiver 10 in applying a precodingoperation at the transceiver 10. That is, in simple terms, the precoderselection feedback 44 can be understood as the second transceiver 12providing dynamically changing precoder recommendations to the firsttransceiver 10, according to changing channel conditions, etc. Thetransceiver 10 considers this precoder information from the secondtransceiver 12 in determining the precoding operation it applies inprecoding the transmissions 46 sent from the first transceiver 10 to thesecond transceiver 12. Also, in one or more embodiments, the transceiver10 sends control signaling 48 to the transceiver 12, to control itsprecoding recommendations.

In at least one embodiment, the control and processing circuits 18 ofthe transceiver 10 at least in part comprise computer-based circuitry,e.g., one or more microprocessors and/or digital signals processors, orother digital processing circuitry. In at least one embodiment, suchcircuitry is specially configured to implement the methods taught hereinfor the transceiver 10, based on executing stored computer programinstructions, such as may be stored in the memory circuit(s) 24.Likewise, in at least one embodiment, the control and processingcircuits 34 are implemented at least in part via programmable digitalprocessing circuitry. For example, the control and processing circuits34 in one or more embodiments include one or more microprocessors ordigital signal processors configured to implement at least a portion ofthe methods taught herein for the transceiver 12, based on executingcomputer program instructions stored in the one or more memory circuits40.

With these example implementation details in mind, in one or moreembodiments, the transceiver 12 is configured to provide precoderselection feedback 44 to the transceiver 10, as precodingrecommendations to the transceiver 10. In support of this configuration,the transceiver 12 includes a receiver (within the transceiver circuits16) that is configured to receive signals from the transceiver 10. Thetransceiver 12 further includes the channel estimator 38, which here isconfigured to estimate channel conditions at the transceiver 12, withrespect to signals received from the transceiver 10. Still further, thetransceiver 12 includes a transmitter (within the transceiver circuits16) that is configured to transmit signals to the transceiver 10,including signals conveying the precoder selection feedback 44.

Additionally, the transceiver 12 includes the previously mentionedprecoding feedback generator 42, which is configured to operate ineither of first and second feedback modes. With reference to FIG. 2, onesees a predetermined set 50 of precoders 28, which are indexed by afull-range index value 52. As an example, the predetermined set 50 ofprecoders 28 represents all of the precoders 28 in the codebook 26 shownin FIG. 1. Of course, that example is non-limiting; the predeterminedset 50 of precoders 28 itself may be a subset within a larger set ofprecoders 28, and one may extend this idea to multiple codebooks 26.Further, the precoders 28 are not necessarily all the same—e.g., theremay be different subsets of precoders 28 for the factorized precoderdiscussed earlier and/or for different operating conditions or modes.

In any case, FIG. 2 further depicts at least one predetermined subset 54of precoders 28 that are indexed by a smaller-range index value 56. Itwill be understood that the predetermined subset 54 of precoders 28 is adefined subset of precoders 28 in the larger set 50 of precoders 28. Asa non-limiting example, assume that the set 50 of precoders 28 includessixteen values indexed as {0, 1, 2, . . . , 15}. Further, assume thatthe subset 54 of precoders 28 corresponds to a subset of eight of thosesixteen precoders and specifically corresponds to those precoders 28 atindex positions {0, 2, 4, 6, . . . , 14} within the larger set 50. Forthis example, subset selections can be signaled using a renumbered indexrange {0, 1, 2, . . . , 7}, which indicates selections from the {0, 2,4, 6, . . . , 14} subset 54 within the larger {0, 1, 2, . . . , 15} set50. Thus, the smaller-range index value 56 need only span the renumberedindex values {0, 1, . . . , 7}.

In one example, the predetermined set 50 of precoders 28 includes anumber of DFT-based precoders representing a total of N DFT-based beamsfor transmission beamforming (at the transceiver 10). Correspondingly,the predetermined subset 54 of precoders 28 represents a total of MDFT-based beams for transmission beamforming and corresponds to everyR-th one of the N DFT-based beams. Here, N, M, and R are integer valuesand M<N.

In at least one such embodiment, at least some of the precoders 28 inthe predetermined set 50 of precoders are based on a factorized precoderdesign comprising a conversion precoder and a tuning precoder. Forexample, the DFT-based precoders in the predetermined set 50 ofprecoders 28 are each formed as the combination of a selected conversionprecoder and a selected tuning precoder. Thus, such precoders 28 in thepredetermined set 50 of precoders 28 correspond to a set of N differentconversion precoders and a set of tuning precoders. Each said conversionprecoder comprises a block diagonal matrix in which each block comprisesa DFT-based precoder that defines N different DFT-based beams for asubgroup in a group of N_(T) transmit antenna ports at the firsttransceiver (10), and the predetermined subset of precoders representsevery R-th one of said N DFT-based beams.

In another example, the set 50 of precoders 28 represents thirty-two(32) DFT-based beams—i.e., N=32. The subset 54 of precoders 28represents a subset of eight of those beams—i.e., M=8. Thus, thesubset-based precoding keeps only eight of the thirty-two beams forantenna subgroup precoding. Moreover, for precoding with the subset ofeight beams, it is contemplated herein to down-sample the QPSK-alphabetphase adjustment between the antenna subgroups to a BPSK alphabet (+−1).More generally, with N beams in the set 50, a subset M of those beamscan be formed by taking every R-th one of the N beams, where M, N, and Rare integers and M<N. The phase resolution that defines the beam phaseoffsets is correspondingly adjusted when switching from the subset 50 ofN precoders 28 corresponding to N DFT-based beams to the subset 54 of Mprecoders 28 corresponding to M DFT-based beams.

Advantageously, because the “index space” spanned by the smaller-rangeindex value 56 is smaller than that of the full-range index value 52,the smaller-range index value 56 can be represented by fewer informationbits than are needed to represent the full-range index value 52. SeeFIGS. 3 and 4 for example illustrations, showing “X” bits representingthe full-range index value 52 and “Y” bits representing thesmaller-range index value 56, where Y<X.

With the above details in mind, the precoding feedback generator 42 atthe transceiver 12 is configured to determine whether to operate in afirst feedback mode, or in a second feedback mode. In the first feedbackmode, the precoder selection feedback 44 is generated using full-rangeindex values 52, while in the second feedback mode, the precoderselection feedback 44 is generated using smaller-range values 56.Sending the precoder selection feedback 44 thus requires a lowersignaling overhead when the precoding feedback generator 42 is operatingin the second feedback mode.

The precoding feedback generator 42 is configured to determine which oneof the two modes it operates in based on, for example, which physicaltransmission channel is being used to transmit the precoding selectionfeedback 44 and/or based on control signaling 48 received from the firsttransceiver 10.

In any case, the precoding feedback generator 42 is configured suchthat, when operating in the first feedback mode, it selects a precoder28 from the predetermined set 50 of precoders 28 based on the channelconditions and sends a full-range index value 52 for the selectedprecoder 28 to the transceiver 10. When operating in the second feedbackmode, the precoding feedback generator 42 is configured to select itsprecoding recommendations from the predetermined subset 54 of precoders28 contained in the predetermined set 50 of precoders 28 based on saidchannel conditions, and to send a smaller-range index value 56, for theselected precoder 28 to the transceiver 10 as the precoder selectionfeedback 44. As such, the transceiver 12 advantageously uses a smallersignaling payload for sending smaller-range index values 56 as comparedto the signaling payload used to send full-range index values 52. Forexample, the full-range index value 52 might be defined as an eight-bitvalue, while the smaller-range index value 56 might be defined as afour- or five-bit value.

FIG. 5 illustrates one example of a method corresponding to the aboveprocessing, where the illustrated method 500 is carried out at thesecond transceiver 12 and provides precoder selection feedback 44, asprecoding recommendations from the transceiver 12 to the transceiver 10.The method 500 includes determining channel conditions at thetransceiver 12 (Block 502). Those of ordinary skill in the art willappreciate, for example, that the transceiver 12 measures channelconditions dynamically with respect to signals received from thetransceiver 10, and assesses those channel conditions as a basis formaking precoder recommendations to the transceiver 10. Further, it willbe understood that the precoder selection feedback 44 may be sent asinformation within a Channel State Information (CSI) report, which mayinclude additional information about reception conditions at thetransceiver 12.

The method 500 continues with determining whether the transceiver 12 isoperating in the first feedback mode or in the second feedback mode(Block 504). If the transceiver 12 is operating in the first feedbackmode, the method 500 continues with the transceiver 12 selecting aprecoder 28 from the predetermined set 50 of precoders 28, asillustrated in FIG. 2, where that selection is based on the channelconditions (Block 506). From there, the method 500 continues withsending the full-range index value 52 for the selected precoder 28 tothe transceiver 10 as the precoder selection feedback 44 (Block 508).

If the transceiver 12 is operating in the second feedback mode (NO fromBlock 504), then the method includes selecting a precoder 28 from thesmaller, predetermined subset 54 of precoders 28, which is containedwithin the predetermined set 50 of precoders 28 (Block 510). From there,the method 500 continues with sending a smaller-range index value 56 forthe selected precoder 28 to the transceiver 10 as the precoder selectionfeedback 44 (Block 512). This signaling is done using a smallersignaling payload than used for sending the full-range index value 52.

In at least one embodiment of the method 500, the transceiver 12 sendsthe precoder selection feedback 44 at certain first times on a controlchannel and at certain second times on a data channel. In particular, inat least one embodiment, the method 500 includes selecting the firstfeedback mode when sending the precoder selection feedback 44multiplexed with data on the same physical channel and selecting saidsecond feedback mode when sending the precoder selection feedback 44 onthe control channel. As one example of this case, the transceivers 10and 12 operate in accordance with 3GPP Long Term Evolution (LTE) airinterface standards. For example, the transceiver 10 is an eNodeB in anLTE network and the transceiver 12 is a mobile terminal or other item ofuser equipment (UE). The control channel in this context comprises theLTE Physical Uplink Control Channel (PUCCH) and the data channelcomprises the LTE Physical Uplink Shared Channel (PUSCH). Thetransceiver 12 can send the precoder selection feedback 44 as controlinformation on the PUCCH, e.g., while operating in the second mode, andcan send the precoder selection feedback 44 multiplexed with data on thePUSCH, e.g., when operating in the first mode.

Additionally, in at least one embodiment of the method 500, using asmaller signaling payload for sending smaller-range index values 56, ascompared to the signaling payload used to send full-range index valuescomprises using a smaller number of bits to represent the smaller-rangeindex values as compared to the number of bits used to represent thefull-range index values. For example, with momentary reference to FIGS.3 and 4, the full-range index values 52 are represented by X informationbits, while the smaller-range index values 56 are represented by Yinformation bits, where Y<X.

Further, in at least one embodiment of the method 500, the transceiver12 dynamically selects either the first or the second feedback modesresponsive to receiving control signaling from the transceiver 10. Inthe same or another embodiment, the transceiver 12 is configured toselect the second feedback mode when sending precoder selection feedback44 as an unscheduled transmission, and to select the first feedback modewhen sending precoder selection feedback 44 as a scheduled transmission.

Further, in at least one embodiment of the method 500, the predeterminedset 50 of precoders 28 comprises a predetermined set of DFT-basedprecoders providing a first spatial resolution for beam forming at thetransceiver 10. Here, the predetermined subset 54 of precoders 28contained within the predetermined set 50 comprises a subset of the sameDFT-based precoders providing a second spatial resolution for the beamforming at the transceiver 10. The second spatial resolution is lowerthan the first spatial resolution. With this arrangement, the secondtransceiver 12 provides higher-resolution beam forming feedback to thetransceiver 10 when the second transceiver 12 operates in the firstfeedback mode, at the expense of signaling overhead. Conversely, thetransceiver 12 provides lower-resolution beam forming feedback to thetransceiver 10 when the transceiver 12 operates in the second feedbackmode, with the advantage of signaling that lower-resolution feedback ata lower signaling overhead. Another point to note with this example, andin a larger sense with respect to the teachings herein, is that thepredetermined subset 54 of precoders 28 may be optimized or designed forparticular transmission modes or operating scenarios, as compared to atleast certain other ones of the precoders 28 in the predetermined set 50of precoders 28.

In at least one embodiment, the predetermined set 50 of precoders 28comprises N precoders 28, wherein N is an integer value, and thepredetermined subset 54 of precoders 28 comprises M of the N precoders,wherein M is an integer value less than N. In particular, in at leastone such embodiment, the M precoders are selected from the N precodersto minimize distances on a Grassmanian manifold between the M precoders.

FIG. 6, with its illustration of an example method 600 performed at thetransceiver 10 turns the discussion toward the complementary operationshappening at that node. That is, it will be understood that thetransceiver 10 is configured to advantageously receive and process(interpret and respond to) the precoding feedback 44 regardless ofwhether it includes full-range index values 52 or smaller-range indexvalues 56. In this regard, it will be understood that the feedbackprocessor 22 of the transceiver 10, as shown in FIG. 1, is particularlyconfigured to process both full-range and smaller-range index values 52and 56.

The method 600 involves the transceiver 10 determining a precoderrecommendation from the second transceiver 12, and it will be understoodthat this method may be carried out repeatedly, such as whenever a newCSI report is received from the transceiver 12. With that in mind, themethod 600 includes receiving precoder selection feedback 44 from thetransceiver 12 (Block 602), and determining whether the precoderselection feedback 44 comprises a first type of precoder selectionfeedback including a full-range index value 52, or a second type ofprecoder selection feedback including a smaller-range index value 54(Block 604). Note that smaller-range index values 56 are signaled by thesecond transceiver 12 using a lower signaling overhead as compared tothat used for signaling full-range index values 52.

If the precoder selection feedback 44 is of the first type, the method600 continues with identifying the precoder recommendation byidentifying a precoder 28 from a predetermined set 50 of precoders 28,as indexed by the full-range index value 52 (Block 606). On the otherhand, if the precoder selection feedback 44 is of the second type, themethod 600 continues with identifying the precoder recommendation byidentifying a precoder 28 from a predetermined subset 54 of precoders28, where that subset 54 is indexed by the smaller-range index value 56(Block 608). Further, the method 600 includes determining a precodingoperation for precoding a transmission 46 to the transceiver 12, basedat least in part on the precoder recommendation (Block 610). In thissense, it will be understood that the transceiver 10 may consider anumber of parameters when determining the precoding to use fortransmitting to the transceiver 12. For example, the transceiver 10 is aMIMO base station supporting a plurality of transceivers 12 (e.g., aplurality of UEs), and it determines precoding based on whether it isoperating in SU-MIMO or MU-MIMO modes and/or based on otherconsiderations, including scheduling load, channel conditions, etc.

In at least one embodiment, the transceiver 10 receives the first typeof precoder selection feedback on a data channel and receives the secondtype of precoder selection feedback on a control channel. In the examplenoted earlier, the transceivers 10 and 12 operate in accordance with LTEair interface standards, and the control channel comprises the PUCCH,and the data channel comprises the PUSCH.

Additionally, or alternatively, in at least one embodiment, thetransceiver 10 sends control signaling 48 to the transceiver 12 tocontrol whether the transceiver 12 sends the first type or the secondtype of precoder selection feedback.

In the same or another embodiment, the predetermined set 50 of precoders28 comprises a predetermined set of DFT-based precoders providing afirst spatial resolution for beam forming at the transceiver 10.Further, the predetermined subset 54 of precoders 28 contained withinthe predetermined set 50 comprises a subset of the same DFT-basedprecoders providing a second spatial resolution for beam forming at thetransceiver 10, with the second spatial resolution being lower than thefirst spatial resolution.

As a more general example, the predetermined set 50 of precoders 28comprises N precoders, wherein N is an integer value, and thepredetermined subset 54 of precoders 28 comprises M of the N precoders,wherein M is an integer value less than N. In at least one case, the Mprecoders are selected from the N precoders to minimize distances on aGrassmanian manifold between the M precoders.

With respect to the method 600 of FIG. 6 and the example functionalcircuit details of FIG. 1, it will be appreciated that, in one or moreembodiments, the transceiver 10 includes a receiver that is configuredto receive the precoder selection feedback 44 from the transceiver 12.For example, the transceiver 16 includes a plurality of radiofrequencyreceivers and transmitters, for communicating with a plurality oftransceivers 12. Further, the transceiver 10 includes a feedbackprocessor 22 configured to determine whether the precoder selectionfeedback 44 comprises a first type of precoder selection feedbackincluding a full-range index value 52, or a second type of precoderselection feedback including a smaller-range index value 56 that issignaled by the second transceiver 12 using a lower signaling overheadas compared to that used for signaling full-range index values.

In the case that the precoder selection feedback 44 is of the firsttype, the feedback processor 22 is configured to identify the precoderrecommendation by identifying a precoder 28 from the predetermined set50 of precoders 28 indexed by the full-range index value 52 included inthe precoder selection feedback 44. In the case that the feedback is ofthe second type, the feedback processor is configured to identify theprecoder recommendation by identifying a precoder 28 from apredetermined subset 54 of precoders 28 indexed by the smaller-rangeindex value 56.

Still further, the feedback processor 22 or the associated precodingcontroller 20 is configured to determine a precoding operation forprecoding a transmission 46 to the second transceiver 12 based at leastin part on the precoder recommendation. See, for example, FIG. 7, whichdepicts a precoding circuit 70 included in the transceiver 16 of thetransceiver 10. The precoder circuit 70 enables the transceiver 10 toprecode transmissions according to an applied precoding operation, andthe transceiver 10 may have more than one such circuit.

According to the example illustration, the precoding circuit 70 receivesinput data, e.g., information symbols to be transmitted, and it includeslayer processing circuit 72 that is responsive to a rank control signalfrom the precoding controller 20. Depending on the transmit rank in use,the input data is placed onto one or more spatial multiplexing layersand the corresponding symbol vector(s) s are input to a precoder 74.

As an example, the precoder 74 is shown as applying an effectiveprecoder W that is formed as the matrix multiplication of a conversionprecoder W^((c)) and a tuning precoder W^((t)). More broadly, theprecoder 74 applies a precoding operation determined by the precodingvalue(s) provided to it by the precoding controller 20. Those values mayor may not follow the recommendations included in the precoder selectionfeedback 44 received from the transceiver 12, but the transceiver 10 atleast considers those recommendations in its precoding determinations.In any case, the precoder 74 outputs precoded signals to Inverse FastFourier Transform (IFFT) processing circuits 76, which in turn providesignals to a number of antenna ports 78 associated with the antennas 14shown in FIG. 1.

Such precoding offers well understood advantages in the context ofwireless communication networks, such as the network 80 illustrated inFIG. 8. Here, the simplified network diagram illustrates a Radio AccessNetwork (RAN) 82, including one or more base stations 84, and anassociated Core Network (CN) 88. This arrangement communicativelycouples user equipment (UE) 86 to other devices in the same networkand/or in one or more other networks. To this end, the CN 88 iscommunicatively coupled to one or more external networks 90, such as theInternet and/or the PSTN.

Of particular interest herein, one sees that the base station 84 storesone or more codebooks 26, as does the UE 86. For purposes of thisexample, the base station 84 comprises an eNodeB or other network basestation and represents the transceiver 10. Similarly, the UE 86represents the transceiver 12. As such, one sees precoded transmissions46 from the base station 84 to the UE 86, along with optional controlsignaling 48 that indicates to the UE 86 whether it should operate inthe first or second feedback mode. Such signaling may be sent usingRadio Resource Control (RRC) signaling, for example.

One also sees the transmission of precoder selection feedback 44 fromthe UE 86 to the base station 84. It will be understood that suchsignaling is dynamically changed from the first type to the second type,based on the feedback mode of the UE 86. It will also be understood thatthe second type of precoder selection feedback 44 requires a lowersignaling overhead because of the smaller size of the smaller-rangeindex values 56, as compared to the full-range index values 52.

With these network-related possibilities in mind, at least oneembodiment of the transceiver 10 uses Discrete Fourier Transform (DFT)based precoders that implement a partially overlapping grid of beams.This approach is suitable for closely spaced co-polarized antennas suchas a Uniform Linear Array with N_(T) elements. Thus, it will beunderstood that in one or more embodiments, the precoders 28 in thecodebook 26 include a number of DFT-based precoders. For example, theprecoders 28 may include a number of DFT-based conversion precoders andassociated tuning precoders.

DFT based precoders are also suitable for the two N_(T)/2 elementantenna group ULAs in a closely spaced cross-pole setup. By a cleverchoice of the codebook entries for the conversion and tuning precodersand exploiting them jointly, the teachings herein ensure re-use of theDFT based size N_(T)/2 precoders for antenna group ULAs also in formingthe needed number of DFT based size N_(T) precoders for an N_(T) elementULA. Moreover, one or more embodiments disclosed herein provide astructure for the conversion precoder that allows re-using existingcodebooks with DFT based precoders and extending their spatialresolution.

Further, in at least one embodiment, it is proposed herein to use aprecoder structure which solves the problems related to PA utilizationand rank nested property for a factorized precoder design—e.g., in thecase where a precoder W is represented in factorized form by aconversion precoder W^((c)) and a tuning precoder W^((t)). By using aso-called double block diagonal tuning precoder combined with a blockdiagonal conversion precoder, full PA utilization is guaranteed and rankoverride exploiting nested property also for the overall precoder ispossible. However, it should be kept in mind that these and otherspecial precoder types and structures may be represented in subsets orgroups within a larger number of precoders 28, and that the codebook 26may include precoders 28 having different structures.

In any case, an example embodiment illustrates re-using DFT basedprecoder elements for an antenna group ULA in a closely spacedcross-pole and also in creating a grid of beams with sufficient overlapfor a ULA of twice the number of elements compared with the antennagroup ULA. In other words, certain precoders 28 in the codebook 26 canbe designed for use with the multiple antennas 14 of the transceiver 10,regardless of whether those antennas 14 are configured and operated asan overall ULA of N_(T) antennas or antenna elements, or as twocross-polarized ULA sub-groups, each having N_(T)/2 antennas or antennalelements.

Consider the block diagonal factorized precoder design given as

$\begin{matrix}{{W = {{W^{(c)}W^{(t)}} = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}W^{(t)}}}},} & (10)\end{matrix}$and note that in order to tailor the transmission to ±45 degreescross-poles, the structure of a conversion precoder can be modified bymeans of a multiplication from the left with a matrix

$\begin{matrix}{\begin{bmatrix}I & {I\;{\mathbb{e}}^{j\;\phi}} \\I & {{- I}\;{\mathbb{e}}^{j\;\phi}}\end{bmatrix},} & (11)\end{matrix}$which, for φ=0, rotates the polarizations 45 degrees to align withhorizontal and vertical polarization. Other values of φ may be used toachieve various forms of circular polarization. Henceforth, it isassumed for purposes of this discussion that such rotations are absorbedinto the channel.

For an N_(T) element ULA, the precoder W for rank 1 is to be a N_(T)×1vector as

$\begin{matrix}{W = {w_{n}^{({N_{T},Q})} = {\begin{bmatrix}w_{1,n}^{({N_{T},Q})} & w_{2,n}^{({N_{T},Q})} & \ldots & w_{N_{T},n}^{({N_{T},Q})}\end{bmatrix}^{T}.}}} & (12)\end{matrix}$In this context, recall that W may be formed as the product (matrixmultiplication) of a given conversion precoder and a correspondingtuning precoder, e.g., W=W^((c))W^((t)). Noting that for antennas m=0,1, . . . , N_(T)/2−1,

$\begin{matrix}{\begin{matrix}{w_{m,n}^{({N_{T},Q})} = {\exp( {j\frac{2\;\pi}{N_{T}Q}{mn}} )}} \\{= {\exp( {j\frac{2\;\pi}{\frac{N_{T}}{2}( {2\; Q} )}{mn}} )}} \\{{= w_{m,n}^{({{N_{T}/2},{2Q}})}},}\end{matrix}{{n = 0},\ldots\mspace{11mu},{{QN}_{T} - 1},}} & (13)\end{matrix}$while for the remaining antennas m=N_(T)/2+m′, m′=0, 1, . . . ,N_(T)/2−1,

$\begin{matrix}{\begin{matrix}{w_{{{N_{T}/2} + m^{\prime}},n}^{({N_{T},Q})} = {\exp( {j\frac{2\;\pi}{N_{T}Q}( {{N_{T}/2} + m^{\prime}} )n} )}} \\{= {{\exp( {j\frac{2\;\pi}{\frac{N_{T}}{2}( {2\; Q} )}m^{\prime}n} )}{\exp( {j\frac{\pi}{Q}n} )}}} \\{= {w_{m^{\prime},n}^{({{N_{T}/2},{2\; Q}})}{\exp( {j\frac{\pi}{Q}n} )}}} \\{{= {w_{m^{\prime},n}^{({{N_{T}/2},{2\; Q}})}\alpha}},}\end{matrix}{{n = 0},\ldots\mspace{11mu},{{QN}_{T} - 1.}}{{Here},{\alpha \in {\{ {{{{\exp( {j\frac{\pi}{Q}n} )}:n} = 0},1,\ldots\mspace{11mu},{{2\; Q} - 1}} \}.}}}} & (14)\end{matrix}$

Any N_(T) element DFT precoder can thus be written as

$\begin{matrix}\begin{matrix}{w_{n}^{({N_{T}Q})} = \lbrack \begin{matrix}w_{0,n}^{({N_{T},Q})} & w_{1,n}^{({N_{T},Q})} & \ldots & w_{{N_{T} - 1},n}^{({N_{T},Q})} & {w_{0,n}^{({N_{T},Q})}\alpha}\end{matrix} } \\ \begin{matrix}{w_{1,2}^{({N_{T}Q})}\alpha} & \ldots & {w_{{N_{T} - 1},n}^{({N_{T},Q})}\alpha}\end{matrix} \rbrack^{T} \\{= \begin{bmatrix}w_{n}^{({{N_{T}/2},{2\; Q}})} \\{w_{n}^{({{N_{T}2},{2\; Q}})}\alpha}\end{bmatrix}} \\{= {{\begin{bmatrix}w_{n}^{({{N_{T}/2},{2\; Q}})} & 0 \\0 & w_{n}^{({{N_{T}/2},{2\; Q}})}\end{bmatrix}\begin{bmatrix}1 \\\alpha\end{bmatrix}}.}}\end{matrix} & (15)\end{matrix}$However, this falls under the factorized precoder structure if thetuning precoder codebook contains the precoder elements

$\begin{matrix}{\{ {{{\begin{bmatrix}1 \\{\exp( {j\frac{\pi}{Q}n} )}\end{bmatrix}:n} = 0},1,\ldots\mspace{11mu},{{2\; Q} - 1}} \},} & (16)\end{matrix}$and moreover suits the closely spaced cross-polarized array perfectlybecause size N_(T)/2 DFT precoders are now applied on each antenna groupULA and the tuning precoder provides 2Q different relative phase shiftsbetween the two orthogonal polarizations. It is also seen how theN_(T)/2 element w_(n) ^((N) ^(T) ^(/2,2Q)) precoders are reused forconstructing the N_(T) element precoder w_(n) ^((N) ^(T) ^(,Q)).

Thus, as an example, the codebook 26 at the transceiver 10 and at thetransceiver 12 may be represented as two codebooks, or two sets ofprecoders 28, with one set comprising conversion precoders W^((c)) andthe other set comprising tuning precoders W^((t)). As for usingfull-range index values 52 or smaller-range index values 56 inaccordance with the teachings herein, it will be understood thatsmaller-range index values 56 may be used to index a subset of theconversion precoders W^((c)) and/or the tuning precoders W^((t)).

Some or all of the conversion precoders W^((c)) are DFT based precoderswith an oversampling factor 2Q, which are used together with at leastsome of the tuning precoders W^((t)) for building DFT based precoders Wwith oversampling factor Q for an antenna array with twice as manyelements. As seen, the oversampling factor Q is now twice as large asfor the co-polarized N_(T) element ULA, but those elements are notwasted because they help to increase the spatial resolution of the gridof beams precoders even further. This characteristic is particularlyuseful in MU-MIMO applications where good performance relies on theability to very precisely form beams towards the UE of interest andnulls to the other co-scheduled UEs.

For example, take a special case of N_(T)=8 transmit antennas—i.e.,assume that the transceiver 10 of FIG. 1 includes eight antennas 14, foruse in precoded MIMO transmissions, and assume that Q=2 for the closelyspaced ULA. One sees that the precoder is built up as

$\begin{matrix}{\begin{matrix}{w_{n}^{({8,2})} = \begin{bmatrix}w_{n}^{({{N_{T}/2},{2\; Q}})} \\{w_{n}^{({{N_{T}2},{2\; Q}})}\alpha}\end{bmatrix}} \\{{= {\begin{bmatrix}w_{n}^{({4,4})} & 0 \\0 & w_{n}^{({4,4})}\end{bmatrix}\begin{bmatrix}1 \\{\exp( {j\frac{\pi}{2}n^{\prime}} )}\end{bmatrix}}},}\end{matrix}{{n = 0},\ldots\mspace{11mu},{{2\; N_{T}} - 1},{n^{\prime} = 0},1,2,3.}} & (17)\end{matrix}$

The codebook entries for the tuning precoders can then be chosen fromthe rank 1, 2 Tx codebook in LTE and hence that codebook can be re-used.The codebook for the conversion precoder contains elements constructedfrom four DFT based generator matrices as in Eq. (8). The codebook 26can contain other elements in addition to the DFT based ones. Broadly,this principle of constructing N element DFT precoders out of smaller,N/2 element DFT precoders can thus be used in general to add efficientclosely spaced ULA and cross-pole support to a codebook based precodingscheme. Advantageously, this particular precoder structure can be usedeven if the antenna setups differ from what is assumed in this example.

Further, note that DFT-based precoders can be used for highertransmission ranks than one as well. One way is to pick the conversionprecoders {tilde over (W)}^((c)) as column subsets of DFT-basedgenerator matrices, such as shown in Eq. (8). The tuning precoders canbe extended with additional columns as well, to match the desired valueof the transmission rank. For transmission rank 2, a tuning precoderW^((t)) can be selected as

$\begin{matrix}{{W^{(t)} = \begin{bmatrix}1 & 1 \\\alpha & {- \alpha}\end{bmatrix}},{\alpha \in {\{ {{{{\exp( {j\frac{\pi}{Q}n} )}:n} = 0},1,\ldots\mspace{11mu},{{2\; Q} - 1}} \}.}}} & (18)\end{matrix}$

It is sometimes beneficial to re-use existing codebooks in the design ofnew codebooks. However, one associated problem is that existingcodebooks may not contain all the needed DFT precoder vectors to provideat least Q=2 times oversampling of the grid of beams. Assuming forexample that one has an existing codebook for N_(T)/2 antennas with DFTprecoders providing Q=Q_(e) in oversampling factor and that the targetoversampling factor for the N_(T)/2 element antenna group ULA isQ=Q_(t). The spatial resolution of the existing codebook can then beimproved to the target oversampling factor in factorized precoder designas

$\begin{matrix}{\mspace{79mu}{{{w = {\begin{bmatrix}{\Lambda_{\overset{\sim}{q}}w_{n}^{({{N_{T}/2},Q_{e}})}} & 0 \\0 & {\Lambda_{\overset{\sim}{q}}w_{n}^{({{N_{T}/2},Q_{e}})}}\end{bmatrix}\begin{bmatrix}1 \\\alpha\end{bmatrix}}},\mspace{79mu}{n = 0},\ldots\mspace{11mu},{{Q_{e}N_{T}} - 1},\mspace{79mu}{\overset{\sim}{q} = 0},1,\ldots\mspace{11mu},{{Q_{t}/Q_{e}} - 1}}{\Lambda_{\overset{\sim}{q}} = {{{diag}( {1,{\exp( {j\frac{2\;\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}1} )},{\exp( {j\frac{2\;\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}2} )},\ldots\mspace{11mu},( {j\frac{2\;\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}( {{N_{T}/2} - 1} )} )} )}.}}}} & (19)\end{matrix}$Here, the w_(n) ^((N) ^(T) ^(/2,Q) ^(e) ⁾ could be elements in theexisting LTE 4 Tx House Holder codebook, which contains 8 DFT basedprecoders (using an oversampling factor of Q=2) for rank 1. When thetransmission rank is higher than one, the block diagonal structure canbe maintained and the structure thus generalizes to

$\begin{matrix}{{W = {\begin{bmatrix}{\Lambda_{\overset{\sim}{q}}{\overset{\sim}{W}}^{(c)}} & 0 \\0 & {\Lambda_{\overset{\sim}{q}}{\overset{\sim}{W}}^{(c)}}\end{bmatrix}W^{(t)}}},} & (20)\end{matrix}$where W is now an N_(T)×r matrix, {tilde over (W)}^((c)) is a matrixwith at least one column equal to a DFT based precoder w_(n) ^((N) ^(T)^(/2,Q) ^(e) ⁾, and the tuning precoder W^((t)) has r columns.

To see that that the spatial resolution can be improved by multiplyingthe antenna group precoder with a diagonal matrix as described above,consider the alternative parameterization of DFT precoders in Eq. (7),

$\begin{matrix}{{w_{m,{{Q_{t}l} + q}}^{({N_{T},Q_{t}})} = {\exp( {j\frac{2\pi}{N_{T}}{m( {l + \frac{q}{Q_{t}}} )}} )}},{m = 0},\ldots\mspace{14mu},{N_{T} - 1},{l = 0},\ldots\mspace{14mu},{N_{T} - 1},{q = 0},\ldots\mspace{14mu},{Q_{t} - 1},} & (21)\end{matrix}$and let

$\begin{matrix}{{q = {{\frac{Q_{t}}{Q_{e}}q^{\prime}} + \overset{\sim}{q}}},{q^{\prime} = 0},\ldots\mspace{14mu},{Q_{e} - 1},{\overset{\sim}{q} = 0},\ldots\mspace{14mu},{\frac{Q_{t}}{Q_{e}} - 1},} & (22)\end{matrix}$to arrive at

$\begin{matrix}{\begin{matrix}{w_{m,{{Q_{t}l} + {\frac{Q_{t}}{Q_{e}}q^{\prime}} + \overset{\sim}{q}}}^{({N_{T},Q_{t}})} = {\exp( {j\frac{2\pi}{N_{T}}{m( {l + {\frac{1}{Q_{t}}( {{\frac{Q_{t}}{Q_{e}}q^{\prime}} + \overset{\sim}{q}} )}} )}} )}} \\{= {{\exp( {j\frac{2\pi}{N_{T}}{m( {l + \frac{q^{\prime}}{Q_{e}}} )}} )}{\exp( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} )}}} \\{= {w_{m,{{Q_{e}l} + q^{\prime}}}^{({N_{T},Q_{e}})}{\exp( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} )}}}\end{matrix}{for}{{m = 0},\ldots\mspace{14mu},{N_{T} - 1},{l = 0},\ldots\mspace{14mu},{N_{T} - 1},{q^{\prime} = 0},\ldots\mspace{14mu},{Q_{e} - 1},{\overset{\sim}{q} = 0},\ldots\mspace{14mu},{\frac{Q_{t}}{Q_{c}} - 1.}}} & (23)\end{matrix}$

The above formulations demonstrate an advantageous aspect of theteachings presented herein. Namely, a codebook containing DFT precoderswith oversampling factor Q_(e) can be used for creating a higherresolution DFT codebook by multiplying the m:th element with

$\exp( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} )$and hence proving that the diagonal transformation given byΛ_({tilde over (q)}) indeed works as intended. It is also conceivablethat such a structure where the antenna group precoder is multipliedwith a diagonal matrix in general (i.e., even when the codebooks are notusing DFT based vectors) can improve the performance.

As for the desirable properties of full PA utilization and rank nestedproperty, a first step in designing efficient factorized precodercodebooks while achieving full PA utilization and fulfilling rank nestedproperty is to make the conversion precoders block diagonal as in Eq.(4). In a particular case, the number of columns k of a conversionprecoder is made equal to 2┌r/2┐, where ┌•┐ denotes the ceil function.This structure is achieved by adding two new columns contributingequally much to each polarization for every other rank. In other words,the conversion precoder W^((c)) at issue here can be written in the form

$\begin{matrix}\begin{matrix}{W^{(c)} = \begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}} \\{{= \begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}},}\end{matrix} & (24)\end{matrix}$where {tilde over (w)}_(l) ^((c)) is an N_(T)/2×1 vector.

Extending the conversion dimension in this manner helps keep the numberof dimensions small and in addition serves to make sure that bothpolarizations are excited equally much. It is beneficial if theconversion precoder, denoted here as {tilde over (W)}^((c)), is alsomade to obey a generalized rank nested property in that there is freedomto choose {tilde over (W)}^((c)) with L columns as an arbitrary columnsubset of each possible {tilde over (W)}^((c)) with L+1 columns. Analternative is to have the possibility to signal the column orderingused in {tilde over (W)}^((c)). Flexibility in the choice of columns for{tilde over (W)}^((c)) for the different ranks is beneficial so as tostill be able to transmit into the strongest subspace of the channeleven when rank override using a column subset is performed.

To ensure full PA utilization, e.g., at the transceiver 10, the tuningprecoders W^((t)) are constructed as follows: (a) the conversion vector{tilde over (w)}_(n) ^((c)) is made constant modulus; and (b) a columnin the tuning precoder has exactly two non-zero elements with constantmodulus. If the m:th element is non-zero, so is element m+┌r/2┐. Hencefor rank r=4, the columns in the tuning precoder are of the followingform

$\begin{matrix}{\begin{bmatrix}x \\0 \\x \\0\end{bmatrix},\begin{bmatrix}0 \\x \\0 \\x\end{bmatrix},} & (25)\end{matrix}$where x denotes an arbitrary non-zero value which is not necessarily thesame from one x to another. Because there are two non-zero elements in acolumn, two orthogonal columns with the same positions of the non-zeroelements can be added before columns with other non-zero positions areconsidered. Such pairwise orthogonal columns with constant modulusproperty can be parameterized as

$\begin{matrix}{\begin{bmatrix}1 \\0 \\{\mathbb{e}}^{j\phi} \\0\end{bmatrix},{\begin{bmatrix}1 \\0 \\{- {\mathbb{e}}^{j\phi}} \\0\end{bmatrix}.}} & (26)\end{matrix}$Rank nested property for the overall precoder is upheld when increasingthe rank by one by ensuring that columns for previous ranks excite thesame columns of the conversion precoder also for the higher rank.Combining this with Eq. (25) and the mentioned pairwise orthogonalproperty of the columns leads to a double block diagonal structure ofthe tuning precoder taking the form

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}x & x & 0 & 0 & \ldots & \; & \; & \; \\0 & 0 & x & x & \; & \; & \; & \; \\\vdots & \; & \; & \; & \ddots & \; & \; & \; \\\; & \; & \; & \; & \; & \; & \; & \; \\x & x & 0 & 0 & \ldots & \; & \; & \; \\0 & 0 & x & x & \; & \; & \; & \; \\\vdots & \; & \; & \; & \ddots & \; & \; & \; \\\; & \; & \; & \; & \; & \; & \; & \;\end{bmatrix}.}}}} & (27)\end{matrix}$Using the pairwise orthogonality property in Eq. (26), and representingthe precoder structure W as W^((c))W^((t)), the precoder structure canbe further specialized into

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}1 & 1 & 0 & 0 & \ldots & \; & \; \\0 & 0 & 1 & 1 & \; & \; & \; \\\vdots & \; & \; & \; & \ddots & \; & \; \\\; & \; & \; & \; & \; & \; & \; \\{\mathbb{e}}^{{j\varphi}_{l}} & {- {\mathbb{e}}^{{j\varphi}_{l}}} & 0 & 0 & \ldots & \; & \; \\0 & 0 & {\mathbb{e}}^{{j\varphi}_{2}} & {- {\mathbb{e}}^{{j\varphi}_{2}}} & \; & \; & \; \\\vdots & \; & \; & \; & \ddots & \; & \; \\\; & \; & \; & \; & \; & \; & \;\end{bmatrix}.}}}} & (28)\end{matrix}$Note that the double block diagonal structure for the tuning precodercan be described in different ways depending on the ordering of thecolumns used for storing the conversion precoders W^((c)) as entries inthe codebook 26. It is possible to equivalently make the tuningprecoders W^((t)) block diagonal by writing

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & 0 & {\overset{\sim}{w}}_{2}^{(c)} & 0 & \ldots & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 \\0 & {\overset{\sim}{w}}_{1}^{(c)} & 0 & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & \ldots & 0 & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}x & x & 0 & 0 & \ldots & \; & 0 & 0 \\x & x & 0 & 0 & \; & \; & \; & \vdots \\0 & 0 & x & x & \ddots & \; & \; & \; \\\vdots & \; & x & x & \; & \; & \; & \; \\\; & \; & 0 & 0 & \ddots & \; & \; & \; \\\; & \; & \vdots & \; & \; & \; & 0 & 0 \\\; & \; & \; & \; & \ddots & \; & x & x \\0 & 0 & 0 & 0 & \ldots & 0 & x & x\end{bmatrix}.}}}} & (29)\end{matrix}$Re-orderings similar to these do not affect the overall precoder W andare thus considered equivalent and assumed to be covered under the terms“block diagonal conversion precoder and double block diagonal tuningprecoder.” It is also interesting to note that if the requirements onthe orthogonality constraint and full PA utilization are relaxed, thedesign for rank nested property can be summarized with the followingstructure for the tuning precoders

$\begin{matrix}{\begin{bmatrix}x & x & x & x & x & x & \; & \; \\0 & 0 & x & x & x & x & \; & \; \\\vdots & \; & \; & \; & x & x & \ddots & \; \\\; & \; & \; & \; & \; & \; & \; & \; \\x & x & x & x & x & x & \; & \; \\0 & 0 & x & x & x & x & \; & \; \\\vdots & \; & \; & \; & x & x & {\ddots\;} & \; \\\; & \; & \; & \; & \; & \; & \; & \;\end{bmatrix}.} & (30)\end{matrix}$Finally, it is worth mentioning that rank nested property can be usefulwhen applied separately to the conversion precoders and the tuningprecoder. Even applying it only to the tuning precoders can help savecomputational complexity, because precoder calculations across ranks canbe re-used as long as the selected conversion precoder W^((c)) remainsfixed.

As an illustrative example for eight transmit antennas 14 at thetransceiver 10, assume that Rank r=1

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & \; \\\; & w_{1}^{(1)}\end{bmatrix}\begin{bmatrix}1 \\{\mathbb{e}}^{{j\varphi}_{k}}\end{bmatrix}}} & (31)\end{matrix}$Rank r=2

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & \; \\\; & w_{1}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 \\{\mathbb{e}}^{{j\varphi}_{k}} & {- {\mathbb{e}}^{{j\varphi}_{k}}}\end{bmatrix}}} & (32)\end{matrix}$Rank r=3

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & \; & \; \\\; & \; & w_{1}^{(1)} & w_{2}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 & 0 \\0 & 0 & 1 \\{\mathbb{e}}^{{j\varphi}_{k}} & {\mathbb{e}}^{{j\varphi}_{k}} & 0 \\0 & 0 & {\mathbb{e}}^{{j\varphi}_{l}}\end{bmatrix}}} & (33)\end{matrix}$Rank r=4

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & \; & \; \\\; & \; & w_{1}^{(1)} & w_{2}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 & 0 & 0 \\0 & 0 & 1 & 1 \\{\mathbb{e}}^{j\;\varphi_{k}} & {- {\mathbb{e}}^{j\;\varphi_{k}}} & 0 & 0 \\0 & 0 & {\mathbb{e}}^{j\;\varphi_{l}} & {- {\mathbb{e}}^{j\;\varphi_{l}}}\end{bmatrix}}} & (34)\end{matrix}$Rank r=5

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & \; & \; & \; \\\; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 \\0 & 0 & 0 & 0 & 1 \\{\mathbb{e}}^{j\;\varphi_{k}} & {- {\mathbb{e}}^{j\;\varphi_{k}}} & 0 & 0 & 0 \\0 & 0 & {\mathbb{e}}^{j\;\varphi_{l}} & {- {\mathbb{e}}^{j\;\varphi_{l}}} & 0 \\0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{m}}\end{bmatrix}}}} & (35)\end{matrix}$

Rank r=6

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & \; & \; & \; \\\; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 \\{\mathbb{e}}^{j\;\varphi_{k}} & {- {\mathbb{e}}^{j\;\varphi_{k}}} & 0 & 0 & 0 & 0 \\0 & 0 & {\mathbb{e}}^{j\;\varphi_{l}} & {- {\mathbb{e}}^{j\;\varphi_{l}}} & 0 & 0 \\0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{m}} & {- {\mathbb{e}}^{j\;\varphi_{m}}}\end{bmatrix}}}} & (36)\end{matrix}$

Rank r=7

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)} & \; & \; & \; & \; \\\; & \; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 1 \\{\mathbb{e}}^{j\;\varphi_{k}} & {- {\mathbb{e}}^{j\;\varphi_{k}}} & 0 & 0 & 0 & 0 & 0 \\0 & 0 & {\mathbb{e}}^{j\;\varphi_{l}} & {- {\mathbb{e}}^{j\;\varphi_{l}}} & 0 & 0 & 0 \\0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{m}} & {- {\mathbb{e}}^{j\;\varphi_{m}}} & 0 \\0 & 0 & 0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{n}}\end{bmatrix}}}} & (37)\end{matrix}$

Rank r=8

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)} & \; & \; & \; & \; \\\; & \; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 \\{\mathbb{e}}^{j\;\varphi_{k}} & {- {\mathbb{e}}^{j\;\varphi_{k}}} & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & {\mathbb{e}}^{j\;\varphi_{l}} & {- {\mathbb{e}}^{j\;\varphi_{l}}} & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{m}} & {- {\mathbb{e}}^{j\;\varphi_{m}}} & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & {\mathbb{e}}^{j\;\varphi_{n}} & {- {\mathbb{e}}^{j\;\varphi_{n}}}\end{bmatrix}}}} & (38)\end{matrix}$The 4 Tx case follows in a similar manner.

As for the use of first and second types of precoder selection feedback44, with the second type having a reduced signaling overhead, considerthe factorized precoder design being used by way of example herein.Namely, the transceiver 12 uses the precoder selection feedback 44 toindicate an overall precoder W to the transceiver 10, as its precodingrecommendation. More particularly, the overall precoder W is representedby the combination of a selected one of the conversion precoders W^((c))in the codebook 26 and a selected one of the tuning precoders W^((t)) inthe codebook 26 (or in another stored codebook). Of course, thetransceiver 10 is configured to understand and process the factorizedprecoder format—i.e., it understands that the recommended overallprecoder W is formed as the matrix multiplication (product) of arecommended conversion precoder W^((c)) and a recommended tuningprecoder W^((t)). As one example, then, the transceiver 12 uses theprecoder selection feedback 44 to indicate its recommendations forconversion and tuning precoders W^((c)) and W^((t)). In so doing, it mayuse full-range index values to indicate conversion precoderrecommendations when operating in the first feedback mode, and may usesmaller-range index values to indicate conversion precoderrecommendations when operating in the second feedback mode.(Additionally, or alternatively, it may use full- and smaller-rangeindex values for indicating its tuning precoder recommendations.) Inthis regard, it will be understood that the second feedback mode entailsthe transceiver 12 selecting precoders from a smaller subset of thedefined conversion precoders W^((c)) and/or from a smaller subset of thedefined tuning precoders W^((c)).

In any case, one has the overall precoder W formed as

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}{W^{(t)}.}}} & (39)\end{matrix}$If the codebook for the antenna group precoder {tilde over (W)}^((c))contains a set of DFT based precoders, then these precoders can besub-sampled by lowering the oversampling factor. This example ofsub-sampling results in only being able to use every K:th beam in thegrid of beams. Sub-sampling of the codebook can also be performed byselecting the M precoders out of N precoders in the codebook whichmaximizes the minimum distance between the selected precoders on theGrassmanian manifold. Distances here can be measured, for example, asChordal distance, projection two-norm distance, or Fubini-Studydistance.

Sub-sampling principles as described above can also be applied to thetuning precoders or to any precoder design. The sub-sampled codebook(s)can then be used on lower payload capable channels, e.g., PUCCH in LTE,while the full codebooks are used on the more capable ones, e.g., PUSCHin LTE.

In other words, taking the conversion precoders W^((c)) as an example,one may assume that there are N conversion precoders W^((c)) overall. Afull-range index value 52 may be used to index this full set ofprecoders, while a smaller-range index value 56 may be used to index asubset of them, e.g., every K:th one of them. Additionally, oralternatively, full-range and restricted range index values can be usedto index all or a sub-sampled set of the tuning precoders W^((t)). Thisapproach can be understood as using subsampling to provide “coarse” CSIreporting on the PUCCH, while providing richer, higher-resolution CSIreporting on the PUSCH. For example, the use of codebook subsetrestrictions allows an LTE eNodeB to configure a UE to only use a subsetof the possible precoders 28 in a codebook 26, for computing andreporting CSI feedback (including the precoder selection feedback 44).

Of course, the teachings herein are not limited to the specific,foregoing illustrations. For example, terminology from 3GPP LTE was usedin this disclosure to provide a relevant and advantageous context forunderstanding operations at the transceivers 10 and 12, which wereidentified in one or more embodiments as being an LTE eNodeB and an LTEUE, respectively. However, the teachings disclosed herein are notlimited to these example illustrations and may be advantageously appliedto other contexts, such as networks based on WCDMA, WiMax, UMB or GSM.

Further, the transceiver 10 and the transceiver 12 are not necessarily abase station and an item of mobile equipment within a standard cellularnetwork, although the teachings herein have advantages in such acontext. Moreover, while the particular wireless network examples givenherein involve the “downlink” from an eNodeB or other network basestation, the teachings presented herein also have applicability to theuplink. More broadly, it will be understood that the teachings hereinare limited by the claims and their legal equivalents, rather than bythe illustrative examples given herein.

What is claimed is:
 1. A method in a second wireless communicationtransceiver of providing precoder selection feedback to a first wirelesscommunication transceiver, as precoding information for said firsttransceiver, wherein said method includes determining channel conditionsat the second transceiver and comprises: when operating in a firstfeedback mode: selecting a precoder from a predetermined set ofprecoders based on said channel conditions wherein at least some of theprecoders in the predetermined set of precoders are based on afactorized precoder design comprising a conversion precoder with a blockdiagonal matrix structure and a tuning precoder with a double blockdiagonal matrix structure, and sending a full-range index value, withthe ability to indicate any of the precoders in the predetermined set,for the selected precoder to said first transceiver as said precoderselection feedback; when operating in a second feedback mode: selectinga precoder from a smaller, predetermined subset of precoders containedwithin the predetermined set of precoders based on said channelconditions wherein the predetermined subset of precoders is based on thefactorized precoder design, and sending a smaller-range index value,with the ability to only indicate precoders within the predeterminedsubset of precoders, for the selected precoder to said first transceiveras said precoder selection feedback; and using a smaller signalingpayload for sending smaller-range index values as compared to thesignaling payload used to send full-range index values.
 2. The method ofclaim 1, wherein the second transceiver sends the precoder selectionfeedback at certain first times on a control channel and at certainsecond times the transceiver on a data channel, and wherein the methodfurther comprises selecting said first feedback mode when sending theprecoder selection feedback multiplexed together with data on the samephysical channel and selecting said second feedback mode when sendingthe precoder selection feedback on the control channel.
 3. The method ofclaim 2, wherein the first and second transceivers operate in accordancewith Long Term Evolution, LTE, air interface standards, and wherein thecontrol channel comprises the LTE Physical Uplink Control Channel,PUCCH, and said data channel comprises the LTE Physical Uplink SharedChannel, PUSCH.
 4. The method of claim 1, further comprising using thesmaller signaling payload to send the smaller-range index values ascompared to the signaling payload used to send the full-range indexvalues comprises using a smaller number of bits to represent thesmaller-range index values as compared to the number of bits used torepresent the full-range index values.
 5. The method of claim 1, furthercomprising dynamically selecting either the first or the second feedbackmodes responsive to receiving control signaling from the firsttransceiver.
 6. The method of claims 1, wherein the predetermined set ofprecoders comprises a predetermined set of DFT-based precoders providinga first spatial resolution for beam forming at the first transceiver,and wherein the predetermined subset of precoders contained within thepredetermined set comprises a subset of the same DFT-based precodersproviding a second spatial resolution for said beam forming at the firsttransceiver, said second spatial resolution being lower than said firstspatial resolution.
 7. The method of claims 1, wherein the predeterminedset of precoders includes a number of DFT-based precoders representing atotal of N DFT-based beams for transmission beamforming, and wherein thepredetermined subset of precoders represents a total of M DFT-basedbeams for transmission beamforming and corresponds to every R-th one ofsaid N DFT-based beams, where N, M, and R are integer values and M<N. 8.The method of claim 7, wherein at least some of the precoders in thepredetermined set of precoders correspond to a set of N differentconversion precoders and a set of tuning precoders, and wherein eachsaid conversion precoder comprises a block diagonal matrix in which eachblock comprises a DFT-based precoder that defines N different DFT-basedbeams for a subgroup in a group of N_(T) transmit antenna ports at thefirst transceiver, and the predetermined subset of precoders representsevery R-th one of said N DFT-based beams.
 9. The method of claim 1,further comprising selecting said second feedback mode when sendingprecoder selection feedback as an unscheduled transmission, andselecting said first feedback mode when sending precoder selectionfeedback as a scheduled transmission.
 10. A wireless communicationtransceiver configured to provide precoder selection feedback to anotherwireless communication transceiver as precoding information for saidother transceiver, said transceiver comprising a receiver configured toreceive signals from said other transceiver and a channel estimatorconfigured to estimate channel conditions at said transceiver withrespect to signals received from said other transceiver, and whereinsaid transceiver comprises: a transmitter configured to transmit signalsto said other transceiver, including signals conveying said precoderselection feedback; and a precoding feedback generator configured todetermine whether to operate in a first feedback mode or a secondfeedback mode and further configured to: when operating in the firstfeedback mode: select a precoder from a predetermined set of precodersbased on said channel conditions wherein at least some of the precodersin the predetermined set of precoders are based on a factorized precoderdesign comprising a conversion precoder with a block diagonal matrixstructure and a tuning precoder with a double block diagonal matrixstructure, and send a full-range index value, with the ability toindicate any of the precoders in the predetermined set, for the selectedprecoder to said other transceiver as said precoder selection feedback;and when operating in the second feedback mode: select a precoder from apredetermined subset of precoders contained in said predetermined set ofprecoders based on said channel conditions wherein the predeterminedsubset of precoders is based on the factorized precoder design, and senda smaller-range index value, with the ability to only indicate precoderswithin the predetermined subset of precoders, for the selected precoderto said other transceiver as said precoder selection feedback; whereinsaid precoding feedback generator is configured to use a smallersignaling payload for sending smaller-range index values as compared tothe signaling payload used to send full-range index values.
 11. Thetransceiver of claim 10, wherein the transceiver is configured to sendsaid precoder selection feedback at certain first times on a controlchannel and at certain second times on a data channel, and wherein saidprecoding feedback generator is configured to select said first feedbackmode when sending said precoder selection feedback multiplexed togetherwith data on the same physical channel and to select said secondfeedback mode when sending said precoder selection feedback on saidcontrol channel.
 12. The transceiver of claim 11, wherein thetransceiver and said other transceiver are configured to operate inaccordance with Long Term Evolution, LTE, air interface standards, andwherein said control channel comprises the LTE Physical Uplink ControlChannel, PUCCH, and said data channel comprises the LTE Physical UplinkShared Channel, PUSCH.
 13. The transceiver of claim 10, wherein thetransceiver is configured to use the smaller signaling payload forsending the smaller-range index values as compared to sending thefull-range index values based on being configured to use a smallernumber of bits to represent the smaller-range index values as comparedto the number of bits used to represent the full-range index values. 14.The transceiver of claim 10, wherein said transceiver is configured todynamically select either the first or the second feedback mode inresponse to receiving control signaling from said other transceiver. 15.The transceiver of claim 10, wherein said predetermined set of precoderscomprises a predetermined set of DFT-based precoders providing a firstspatial resolution for beam forming at said other transceiver, andwherein the predetermined subset of precoders contained within saidpredetermined set comprises a subset of the same DFT-based precodersproviding a second spatial resolution for said beam forming at saidother transceiver, said second spatial resolution being lower than saidfirst spatial resolution.
 16. The transceiver of claim 10, wherein thepredetermined set of precoders includes a number of DFT-based precodersrepresenting a total of N DFT-based beams for transmission beamforming,and wherein the predetermined subset of precoders represents a total ofM DFT-based beams for transmission beamforming and corresponds to everyR-th one of said N DFT-based beams, where N, M, and R are integer valuesand M<N.
 17. The transceiver of claim 16, wherein at least some of theprecoders in said predetermined set of precoders correspond to a set ofN different conversion precoders and a set of tuning precoders, andwherein each said conversion precoder comprises a block diagonal matrixin which each block comprises a DFT-based precoder that defines Ndifferent DFT-based beams for a subgroup in a group of N_(T) transmitantenna ports at the first transceiver, and the predetermined subset ofprecoders represents every R-th one of said N DFT-based beams.
 18. Thetransceiver of claim 10, wherein said transceiver is configured toselect said second feedback mode when sending said precoder selectionfeedback as an unscheduled transmission, and to select said firstfeedback mode when sending said precoder selection feedback as ascheduled transmission.
 19. A method at a first wireless communicationtransceiver of determining a precoder recommendation from a secondwireless communication transceiver, said method comprising: receivingprecoder selection feedback from the second transceiver; determiningwhether the precoder selection feedback comprises a first type ofprecoder selection feedback including a full-range index value, or asecond type of precoder selection feedback including a smaller-rangeindex value that is signaled by the second transceiver using a lowersignaling overhead as compared to that used for signaling full-rangeindex values; if the precoder selection feedback is the first type ofprecoder selection feedback, identifying the precoder recommendation byidentifying a precoder from a predetermined set of precoders indexed bythe full-range index value included in the precoder selection feedback,wherein at least some of the precoders in the predetermined set ofprecoders are based on a factorized precoder design comprising aconversion precoder with a block diagonal matrix structure and a tuningprecoder with a double block diagonal matrix structure; if the precoderselection feedback is the second type of precoder selection feedback,identifying the precoder recommendation by identifying a precoder from apredetermined subset of precoders indexed by the smaller-range indexvalue included in the precoder selection feedback, said predeterminedsubset of precoders including at least one precoder based on thefactorized precoder design contained within the predetermined set ofprecoders; and determining a precoding operation for precoding atransmission to the second transceiver based at least in part on theprecoder recommendation.
 20. The method of claim 19, further comprisingreceiving said first type of precoder selection feedback on a datachannel and receiving said second type of precoder selection feedback ona control channel.
 21. The method of claim 20, wherein the first andsecond transceivers operate in accordance with Long Term Evolution, LTE,air interface standards, and wherein said control channel comprises theLTE Physical Uplink Control Channel, PUCCH, and said data channelcomprises the LTE Physical Uplink Shared Channel, PUSCH.
 22. The methodof claim 19, further comprising sending control signaling to the secondtransceiver to control whether the second transceiver sends the firsttype or the second type of precoder selection feedback.
 23. The methodof claim 19, wherein said predetermined set of precoders comprises apredetermined set of DFT-based precoders providing a first spatialresolution for beam forming at said first transceiver, and wherein thepredetermined subset of precoders contained within said predeterminedset comprises a subset of the same DFT-based precoders providing asecond spatial resolution for said beam forming at said firsttransceiver, said second spatial resolution being lower than said firstspatial resolution.
 24. The method of claim 19, wherein saidpredetermined set of precoders represents a total of N DFT-based beamsfor transmission beamforming by said transceiver, and wherein thepredetermined subset of precoders represents a total of M DFT-basedbeams for transmission beamforming and corresponds to every R-th one ofsaid N DFT-based beams, where N, M, and R are integer values and M<N.25. The method of claim 24, wherein said M precoders are selected fromthe N precoders to minimize distances on a Grassmanian manifold betweenthe M precoders.
 26. A wireless communication transceiver configured todetermine a precoder recommendation from another wireless communicationtransceiver, said transceiver comprising: a receiver configured toreceive precoder selection feedback from the other transceiver; afeedback processor configured to determine whether the precoderselection feedback comprises a first type of precoder selection feedbackincluding a full-range index value, or a second type of precoderselection feedback including a smaller-range index value that issignaled by the other transceiver using a lower signaling overhead ascompared to that used for signaling full-range index values; saidfeedback processor configured to: if the precoder selection feedback isthe first type of precoder selection feedback, identify the precoderrecommendation by identifying a precoder from a predetermined set ofprecoders indexed by the full-range index value included in the precoderselection feedback, wherein at least some of the precoders in thepredetermined set of precoders are based on a factorized precoder designcomprising a conversion precoder with a block diagonal matrix structureand a tuning precoder with a double block diagonal matrix structure; andif the precoder selection feedback is the second type of precoderselection feedback, identify the precoder recommendation by identifyinga precoder from a predetermined subset of precoders indexed by thesmaller-range index value included in the precoder selection feedback,said predetermined subset of precoders including at least one precoderbased on the factorized precoder design contained within thepredetermined set of precoders; and wherein said feedback processor oran associated precoding controller is configured to determine aprecoding operation for precoding a transmission to the othertransceiver based at least in part on the precoder recommendation. 27.The transceiver of claim 26, wherein said transceiver is configured toreceive the first type of precoder selection feedback over a datachannel between the transceivers, and is configured to receive thesecond type of precoder selection feedback over a control channelbetween the transceivers.
 28. The transceiver of claim 27, wherein saidtransceiver is an eNodeB configured for operation a wirelesscommunication network based on 3GPP Long Term Evolution (LTE) standards,and wherein the data channel comprises the LTE Physical Uplink SharedChannel, PUSCH, and the control channel comprises the LTE PhysicalUplink Control Channel (PUCCH).
 29. The transceiver of claim 26, whereinthe predetermined set of precoders comprises a predetermined set ofDFT-based precoders providing a first spatial resolution for beamforming at said transceiver, and wherein the predetermined subset ofprecoders contained within the predetermined set comprises a subset ofthe same DFT-based precoders providing a second spatial resolution forsaid beam forming at said transceiver, said second spatial resolutionbeing lower than said first spatial resolution.
 30. The transceiver ofclaim 26, wherein the predetermined set of precoders represents a totalof N DFT-based beams for transmission beamforming by said transceiver,and wherein the predetermined subset of precoders represents a total ofM DFT-based beams for transmission beamforming by said transceiver andcorresponds to every R-th one of said N DFT-based beams, where N, M, andR are integer values and M<N, and wherein said other transceiver usessaid smaller-range index values to indicate precoder selections fromamong the M precoders in the predetermined subset and uses saidfull-range index values to indicate precoder selections from among the Nprecoders in the predetermined set.